1. Field of the Invention
The present invention relates to an adsorbent, a method and an activated carbon canister for removing organic compounds from a vaporous stream.
2. Background of the Invention
The use of activated carbon as an adsorbent is one of the most effective and economical methods of controlling emissions of volatile organic compounds such as, for example, benzene, toluene, ketone, gasoline and carbon tetrachloride. Volatile organic compounds are attracted to and held by the activated carbon. Activated carbon's effectiveness as an adsorbent is often attributed to the presence of a vast infrastructure of internal pores, which give rise to a desirably large surface area per unit mass of activated carbon.
Activated carbon is a microcrystalline, nongraphitic form of carbon which has been subjected to an activation process to develop internal porosity. Almost any carbonaceous material, including sawdust, nutshells, coal and petroleum residues, can be utilized as the source of carbon. The carbon can be chemically activated, as by contact with phosphoric acid, zinc chloride, potassium sulfide, potassium hydroxide or the like. Alternatively, the carbon can be activated by contact with, for example, heated air, steam or carbon dioxide.
Activated carbon employed in vapor-phase adsorption typically has a surface area of about 1000 to about 2000 square meters per gram. Most of this surface area is located in pores three nanometers or less in size. Activated carbon for vapor-phase adsorption is commonly composed of porous particles having an effective diameter of about one to about five millimeters. The particles may contain impurities such as moisture or ash.
Organic compounds have been successfully separated from vapor streams by exposure to activated carbon. For example, the petroleum and paint industries employ granular activated carbon to remove volatile organic compounds from process vents and sewer vents. However, if oxygen is present in such a vapor stream, for example, as a component of air in the vapor stream, accidental ignition of the organic compound or the activated carbon is a real danger. The problem of accidental ignition is exacerbated when adsorption of the organic compound on the activated carbon is accompanied by a significant release of heat energy, termed heat of adsorption.
Practitioners have long recognized the need to reduce the likelihood of accidental ignition in vapor phase activated carbon adsorption. For example, some practitioners have attempted to make activated carbon adsorption safer by diluting the vapor phase with a relatively inert ingredient, such as nitrogen or water. However, diluting the vapor phase is not a panacea because diluting the vapor phase tends to reduce adsorption efficiency. Additionally, while diluting the vapor phase may reduce the possibility of accidental ignition to some extent, it cannot entirely eliminate the possibility.
Other practitioners have attempted to make vapor phase activated carbon adsorption safer by wetting with water all or a portion of the activated carbon. However, wetting all of the activated carbon impairs the active carbon's capacity for adsorbing an organic carbon compound from the vapor phase. Moreover, as demonstrated below in Example 2, wetting only a portion of the activated carbon fails to reduce the risk of accidental ignition.